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Using the AD844 as an I/V
Using the AD844 as an I/V
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Old 22nd June 2018, 10:11 PM   #2231
Hierfi is offline Hierfi  Canada
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Hi Joe,
My reason for asking is to determine if the AD844's input stage has enough class A current capability that it can be used as an output buffer in lieu of a BUF03 or the AD844's normal output buffer. In other words using only the input stage of a second AD844 with input to pin 3 and the output taken from pin 2, with its Tz pin 5 connected to ground.

From the thread the output buffer in the AD844 isn't necessarily a good one. Having done experiments with and without the AD844's output buffer it appears susceptible to oscillation of the kind that can become rectified and reflected back into the audio range. Given that oscillations normally occur under feedback conditions it would seem that the current limiters in the output buffer can be responsible. In other words, if high frequency transients exist at the Tz node, as either coming from the input, from the power supplies, or from ground through the RC network that members are connecting to this node, this can affect or activate the current limiters. This necessitates a requirement for good power supply decoupling and general high frequency network considerations in any design. Ultimately the output buffer in the AD844 is a feedback amplifier.

The AD844 has already proven itself as an excellent device by members for use as an I/V converter, one that uses a unity gain input buffer. Given that the input buffer doesn't employ current limiting (as indicated by the data sheet warning not to exceed 5mA on the inverting terminal) it isn't considered prone to oscillations.

Another factor is that the input buffer is designed to be clean (being laser trimmed as well), as the voltage across the input terminals represent the reference points that generates feedback, hence are outside the feedback loop (the AD844 can't guess what needs fixing). This is not the case for the class AB output buffer, normally current limited to protect the device from damage, with feedback limiting its non-linearity.

Cheers, Gerrit
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Old 10th February 2019, 06:48 AM   #2232
raptorlightning is offline raptorlightning
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Quick couple of questions on the 2019 status of this amazingly interesting thread:

Are the newest designs still using a JFET current source circuit on the input? Is this necessary if the output is capacitively coupled? Would modern, in-stock JFETs like the MMBF4393 or 2SK3666 be appropriate since 2SK170s are somewhat expensive at this point?

If one were to forego the OPA627, would a BUF634 be acceptable for a TZ-output arrangement since BUF03's are now also very expensive? I know it's bipolar, but the datasheet says 80Mohm input impedance or 8Mohm in high-BW mode. 80M in parallel with 1.5kohm shouldn't cause much of an issue, right?

I fear the 844's output buffer might not be able to drive much capacitance, especially the ~100pF/m of some RCA cables and that could be causing some of the sound quality issues seen here using pin 6 directly.

Last edited by raptorlightning; 10th February 2019 at 07:02 AM.
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Old 10th February 2019, 06:57 AM   #2233
georgehifi is offline georgehifi  Australia
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Quote:
Originally Posted by raptorlightning View Post
I fear the 844's output buffer might not be able to drive much capacitance, especially the ~100pF/m of some RCA cables and that could be causing some of the sound quality issues seen here using pin 6 directly.
Forget the AD844's own buffer it's not very good sounding. Come off the 844's pin5 to your own buffer, I found a "genuine" OPA627 to be exemplary, make sure it's genuine by getting it from an authorized TI distributor, as there are many fakes of these.

Forget the BUF03 in the pic that's the first one I tried, too dirty sounding great bass though.

Cheers George
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File Type: jpg ad844_c-b_i-v_sch PCM1704 output pin 5.jpg (70.5 KB, 181 views)
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Last edited by georgehifi; 10th February 2019 at 07:02 AM.
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Old 10th February 2019, 07:23 AM   #2234
raptorlightning is offline raptorlightning
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George,

Are we still using the input bias current source? I noticed your recent schematics (including the one you just posted) don't have one.
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Old 10th February 2019, 08:24 AM   #2235
georgehifi is offline georgehifi  Australia
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Quote:
Originally Posted by raptorlightning View Post
George,

Are we still using the input bias current source? I noticed your recent schematics (including the one you just posted) don't have one.
With the above 844 using pin 5 and the OPA627 and small amount dc offset can be trimmed out using the OPA627's offset adjustment.

But if you have a large amount of dc offset coming from the dac thewn you need to use this setup to trim it out.

Cheers George
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File Type: jpg ad844 IV for PCM1704 or PCM1702 with offset nuller.jpg (81.5 KB, 168 views)
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Old 10th February 2019, 05:04 PM   #2236
diyiggy is offline diyiggy
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Default Buffer

Thanks for that tip


If the BUF03 is not good enough, what could be today a swap of the venerable SK170 ? (in case one 'll not want to use feedback witch a 8 pins IC).
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Old 11th February 2019, 04:21 AM   #2237
abraxalito is offline abraxalito  United Kingdom
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If its a line level application then presumably noise isn't a major issue, in which case a SSM3K35 will out-perform an SK170 in practically every other electrical parameter besides noise, breakdown voltage and threshold voltage stability. Its a MOSFET (enhancement mode) though, not a JFET so will need circuitry changes. It is a lot cheaper than a K170 but only available in SMT.
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Old 11th February 2019, 06:39 AM   #2238
diyiggy is offline diyiggy
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Many thanks Abraxilato,


A DAC/CD player unity gain Mosfet output stage could be a winner indeed...
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Old 11th February 2019, 07:00 AM   #2239
abraxalito is offline abraxalito  United Kingdom
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If anyone's interested in a DAC unity gain buffer, I use this one on my 'lingDAC' design. Its using another modern Toshiba MOSFET, SSM3J327 which gives it a very low Zout for a source follower because the FET's transconductance is so high. Mouser has them in stock right now but they do tend to sell out fast....

SSM3J327R,LF Toshiba | Mouser
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File Type: pdf Schematics 0DAC-SEBUFF (1).pdf (28.0 KB, 52 views)
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Old 11th February 2019, 08:11 AM   #2240
kazap is offline kazap
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Quote:
Originally Posted by abraxalito View Post
If anyone's interested in a DAC unity gain buffer....

Thanks very much.
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